1. Field of the Invention
This invention relates to a method and apparatus for selecting the momentum or energy resolution of a charged-particle momentum or energy analyzer which relies on the dispersion of a charged-particle beam and which uses a resolving aperture to define the resolution. In particular, the present invention relates to a method and apparatus for selecting the resolution of a magnetic sector mass spectrometer.
2. Description of the Prior Art
Conventional charged-particle beam energy or momentum analyzers (eg, electrostatic sector or magnetic sector analyzers) cause dispersion of the beam along a dispersion axis, so that the energy or momentum resolution may be determined by the width along that axis of a resolving aperture through which particles having an energy or momentum within a certain range may pass to a detector. Most such analyzers also possess focusing properties along the dispersion axis such that there exists an image plane in which an image of an object (typically defined by an entrance aperture located in an object plane) is formed. Conventionally, the resolving aperture is located in the image plane, resulting in the maximum possible transmission for a particular resolution.
In such a conventional analyzer, varying the width of the resolving aperture changes the resolution of the analyzer (at least within certain limits), but as the width is reduced to increase the resolution the number of charged particles passing through the aperture also may be reduced. Many analyzers of this type therefore incorporate a variable-width resolving aperture (usually a slit) in order to provide adjustable resolution. This allows the analyzer to be operated at either high resolution with low transmission or at low resolution with high transmission. In many cases a continuously adjustable slit is provided, especially in mass spectrometers incorporating magnetic and/or electrostatic sector analyzers. Usually, a mechanism is provided which allows the width of the slit to be adjusted from outside the vacuum envelope of the analyzer.
Many such slit-adjusting mechanisms are known (see, for example, U.S. Pat. Nos. 4,612,440, 3,655,963, 3,546,450 and 3,187,179). Mechanisms which allow both the width and the position of the aperture along the dispersion axis to be adjusted are also known, for example, U.S. Pat. No. 4,213,051. All such adjustable aperture mechanisms involve either a mechanical linkage which transfers motion from outside the vacuum housing to the aperture jaws or an electrical transducer which converts an electrical signal directly into the jaw movement (eg, a bimetallic strip or a piezoelectric device). It is also known to provide several apertures of different sizes, for example in a sliding plate arranged so that any selected one of the apertures can be brought into use by moving the plate. See, for example, U.S. Pat. No. 4,595,831 which discloses such an arrangement for use in a multi-collector isotope-ratio mass spectrometer. In this spectrometer, each position of the plate brings several apertures into use simultaneously, each aligned with a collector which therefore receives ions of a particular mass-to-charge ratio. All these prior resolution adjusting or selecting systems require sliding or rotating parts operating in high vacuum, and although the prior devices have been developed to such a degree that regular use is possible without repeated failures, the difficulty of achieving reliable operation is considerable. Further, the speed at which the width of the aperture can be changed is inherently limited, even when the mechanism is driven by a solenoid or motor. Several attempts have therefore been made to provide an aperture of variable effective width which does not involve moving components, (see, for example, the zoom electrostatic lens arrangement operated in conjunction with a fixed aperture described in GB patent 1,318,200). Unfortunately, use of such a system often results in an increase in focusing aberrations which can limit the ultimate resolution of the analyzer.
Also relevant to this invention-is a prior detector system for a mass spectrometer described in the brochure "Finnigan MAT MAT9OO Mass Spectrometer", published 1989 by Finnigan MAT, W.Germany. This detector system incorporates an electron multiplier type detector operated in conjunction with a single fixed resolving aperture and a position sensitive multichannel detector. The two detectors are selected by electrostatically deflecting the beam towards the selected detector. It does not provide a selectable resolution spectrometer or analyzer which is the object of the present invention, but rather a means of selecting two different detectors.